1. Field of the Invention
This invention relates to production of alumina from bauxite or other aluminum-containing ores. More particularly, this invention relates to the removal of sodium oxalate compounds from Bayer-type spent liquor.
2. Brief Description of the Prior Art
Aluminum-bearing ores, such as bauxite, used in the production of aluminum hydroxide, contain organic materials which are a composite of many substances described by such terms as humic, lignin, cellulose and protein. In caustic digestion processing, commonly known as Bayer processing, a portion of this organic matter is extracted into the liquor during digestion and is decomposed to form soluble sodium organic compounds. One of these degradation products has been identified as sodium oxalate. With recycling (for economic reasons) of the liquor after precipitation of aluminum hydroxide, the sodium oxalate concentration increases until an equilibrium concentration is reached.
Although it is known that pure sodium oxalate in the usual concentration of Bayer spent liquor, i.e., 180 grams/liter total caustic (TC), has little effect on the precipitation of aluminum hydroxide, plant oxalate, that is, oxalate which accumulates in the ore refining process and which is defined to include degraded organic materials, has been blamed for inhibiting precipitation of aluminum hydroxide. When this plant sodium oxalate concentration builds up to a critical supersaturated concentration, then it coprecipitates with the aluminum hydroxide giving rise to aluminum hydroxide fines and interfering with agglomeration of the aluminum hydroxide fines in the precipitation process. This causes serious problems in separating the fine aluminum hydroxide crystals from the mother liquor, resulting in more fine aluminum hydroxide being recycled back to digestion thereby decreasing the overall aluminum hydroxide yield. In order to obtain aluminum hydroxide product, having large particles formed, at least in part, by means of agglomeration, it is necessary to remove at least a portion of the sodium oxalate from the liquor stream. This reduces the accumulation of sodium oxalate and depresses its coprecipitation with aluminum hydroxide crystals in solution.
Several methods have been proposed to reduce the sodium oxalate concentration in caustic liquor. Byrns U.S. Pat. No. 3,337,305 proposes addition of ammonia to a caustic liquor containing oxalates to cause a precipitation of sodium oxalate salts. Yamada et al U.S. Pat. No. 3,899,571 provides for the removal of sodium oxalate by the addition of sodium oxalate seed crystals to a spent liquor to precipitate organic substances, mainly composed of sodium oxalate, which are then removed from the spent liquor.
Breteque U.S. Pat. No. 3,457,032 discloses a process for the purification of sodium aluminate obtained by alkaline digestion of bauxite which comprises treating the solution with strongly basic anion exchange resins of a macroreticular type to eliminate metal ion and organic acid impurities. Lever U.S. Pat. No. 4,275,042 describes the removal of sodium oxalate from Bayer spent liquor by treating the liquor with a cationic sequestrant which interacts with the humic material in the spent liquor to remove it and thereby destabilize the solution with respect to precipitation of sodium oxalate. The patentees also used seed crystals to expedite the precipitation of the sodium oxalate after the removal of the humic material. Gnyra U.S. Pat. No. 4,275,043 describes the removal of oxalate, generally as a disodium salt, from Bayer process liquor by treating the liquor with an adsorbent, such as activated carbon, to remove sufficient of the humic matter in the liquor to destabilize the sodium oxalate and cause it to precipitate.
Sato et al U.S. Pat. No. 3,649,185 describes a process for the removal of sodium oxalate from a spent liquor by increasing the caustic concentration of the liquor as well as controlling the temperature to develop supersaturation. The patentees further point out that the crystallized sodium oxalate is contained mainly in the finer part of the crystals and that this is used for further separation of the sodium oxalate.
While each of these methods has met with some limited success, there still remains a need for an economical method of removing sodium oxalate while minimizing excess process steps and alumina losses.